Known hitherto as catalysts for the preparation of .alpha.-olefin polymers, e.g. ethylene polymer or ethylene/.alpha.-olefin copolymers, are titanium based catalysts composed of titanium compound and organoaluminum compounds, or vanadium based catalysts composed of vanadium compounds and organoaluminum compounds.
Recently, novel Ziegler type catalysts composed of zirconium compounds and organoaluminum oxy compounds have been developed as catalysts for the preparation of ethylene/.alpha.-olefin copolymers in high polymerization activity, and processes for the preparation of ethylene/.alpha.-olefin copolymers using these novel catalysts have been proposed, for example, in Japanese Patent L-O-P Publns. Nos. 19309/1983, 35005/1985, 35006/1985, 35007/1985, 35008/1985, etc.
These novel catalysts formed from the transition metal compounds and organoaluminum oxy compounds as proposed in the prior art are excellent in polymerization activity especially ethylene polymerization activity in comparison with the catalysts formed from transition metal compounds and organoaluminum compounds which have been known prior to the advent of the above-mentioned novel catalysts. Nevertheless, the majority of these novel catalysts are soluble in the reaction system and, in most cases, processes for the preparation of ethylene/.alpha.-olefin copolymers using these novel catalysts are applicable only to the solution polymerization system. When it is intended to prepare polymers high in molecular weight by using these catalysts, there is brought about such inconvenience that a markedly increased viscosity of the solution containing the resulting polymer will reduce productivity of the process, or there is involved such a problem that the resulting polymers obtained by after-treatment have low balk density and it becomes difficult to obtain spherical olefin polymers excellent in particle characteristics.
In the meantime, attempts have been made to polymerize olefins in the suspension or vapor phase polymerization system by using a catalyst composed of a transition metal compound and an organoaluminum oxy compound, at least one of the compounds has been supported on a porous inorganic oxide carrier such as silica, alumina and silica alumina.
For example, the above-mentioned Japanese Patent L-O-P Publns. Nos. 35006/1985, 35007/1985 and 35008/1985 describe to the effect that catalysts prepared by supporting transition metal compounds and organoaluminum oxy compounds on silica, alumina or silica alumina are useful in the preparation of ethylene/.alpha.-olefin copolymers.
Japanese Patent L-O-P Publications Nos. 106808/1985 and 106809/1985 disclose processes for the preparation of compositions containing polyethylene polymers and fillers, wherein ethylene is polymerized or ethylene and other .alpha.-olefin are copolymerized in the presence of a product obtained previously by contacting a highly active catalyst component containing a hydrocarbon-soluble titanium compound and/or zirconium compound with a filler, an organoaluminum compound and a filler which has affinity with polyolefins.
Japanese Patent L-O-P Publn. No. 31404/1986 discloses a process for the polymerization of ethylene or copolymerization of ethylene and other .alpha.-olefin in the presence of a mixed catalyst comprising a product obtained by reaction of trialkylaluminum with water in the presence of silicon dioxide or aluminum oxide, and a transition metal compound.
Japanese Patent L-O-P Publn. No. 276805/1986 discloses polymerization of olefin in the presence of a catalyst comprising a zirconium compound and a reaction mixture obtained by reacting a reaction mixture resulting from aluminoxane and trialkylaluminum with an inorganic oxide having a surface hydroxyl group such as silica.
Japanese Patent L-O-P Publns. Nos. 108610/1986 and 296008/1986 disclose processes for the polymerization of olefin in the presence of a catalyst having supported a transition metal compound such as metallocene and aluminoxane on a carrier such as inorganic oxide.
However, when olefins were polymerized or copolymerized in the suspension or vapor phase polymerization system in the presence of such solid catalyst components supported by carriers as mentioned above, the polymerization activity attained in these system was very low in comparison with the case of the above-mentioned solution polymerization system, and a bulk density of the polymer thereby formed was not sufficiently satisfactory.
Japanese Patent L-O-P Publin. No. 280703/1988 discloses prepolymerization of olefin in the presence of a zirconosene compound, aluminoxane, an organoaluminum compound and a carrier such as silica. In this process, though the polymerization activity is high and the particle characteristics of the polymer thereby formed are good, there is brought about such inconvenience that the prepolymerization catalysts adhere to a wall of a reactor when the prepolymerization is carried out.
Further, olefin polymers prepared by using the catalysts in prior art usually have narrow molecular weight distribution and the catalysts have been used for obtaining such the polymers as having narrow molecular weight distribution. However, the olefin polymers are sometimes limited in the molding conditions when they have narrow molecular weight distribution and, therefor, the polymers are required to have broad molecular weight distribution according to the process by which they are molded or to the purpose for which they are used.
Moreover, when an inflation film is molded at a high speed from the polymer as formed, the polymer used for must be selected from among those having a large melt tension considering their molecular weight in order to carry out a stable molding operation at a high speed without flickering or tearing the bubble. Similar characteristics are necessary for the polymer at the time of blow molding thereof in order to prevent the sagging or tearing-off of the polymer or at the time of T-die extrusion thereof in order to keep the shortage in breadth of film to a minimum.